1. Field of the Invention
The present invention relates generally to amplifiers and more specifically to output stages of operational amplifiers.
2. Description of the Related Art
Generally, an operational amplifier (op amp) is used in many applications which include, but are not limited to, radiotelephones. In fact, current radiotelephones include multiple op amps for increasing signal power in both the receiver and the transmitter. Such op amps are typically mounted, along with other circuitry, on a substrate of an integrated circuit (IC). As radiotelephones become smaller and more portable, there is a growing need for the op amps to operate effectively at low power supply voltages.
Op amps consist of an input stage, an intermediate stage, and an output stage. When the op amp is mounted on an IC at a signal output thereof, it is necessary for the output stage of the op amp to provide a low impedance output. The low impedance output prevents the remainder of the circuit mounted on the IC from becoming unstable from large amounts of capacitance provided by other directly coupled ICs or other components. Unfortunately, many of the existing op amp output stages, including those with the capability of swinging rail-to-rail, exhibit a high output impedance.
FIG. 1 is an illustration in schematic form of a known low impedance output stage for an op amp comprising an emitter (or source) follower buffer 100. The buffer 100 is powered by a first supply voltage rail (+V.sub.BB) 102 and a second supply voltage rail 104. The buffer 100 includes an input 106 for receiving an input voltage 105. The input 106 is coupled to first and second buffering devices 108, 110 via respective first and second biasing devices 112, 114. In response to the input voltage 105, the first and second buffering devices 108, 110 provide an output voltage 115 and an output current 117, collectively an output signal, at an output 116 coupled thereto. The output signal is capable of driving a load, such as another IC, (not shown) that is coupled to the output 116 and similarly powered by the first and second supply voltage rails 102, 104.
FIG. 2 is an illustration in graph form of a known voltage transfer characteristic 200 of the buffer 100 of FIG. 1. In response to the input voltage 105, the first and second buffering devices 108, 110 alternate operation to provide the output signal. As the input voltage 105 exceeds +V.sub.BB /2, as designated by portion 202 of the transfer characteristic 200, the output current 117 is primarily sourced to the output 116 of FIG. 1 by the first buffering device 108. As the input voltage 105 falls below +V.sub.BB /2, as designated by portion 204 of the transfer characteristic 200, the output current 117 is primarily sunk from the output 116 by the second buffering device 110. The first and second biasing devices 112, 114 ensure that the first and second buffering devices 108, 110, respectively, remain on to prevent crossover distortion as the input voltage 105 approximately equals, or passes through, +V.sub.BB /2, as designated by point 206 on the transfer characteristic 200.
Due to base-emitter junction electrical limitations of the bipolar junction transistors comprising the first and second biasing devices 108, 110, the buffer 100 is unable to provide the output signal and drive the load over a maximum voltage range 208. The maximum voltage range 208 is defined by the difference between the first and second supply voltage rails 102, 104 and designated as 0 V to +V.sub.BB. In fact, the buffer 100 is unable to provide the output signal when the input voltage 105 is within a diode drop of either the first or second supply voltage rail 102, 104. A diode drop is commonly known as the voltage drop between the base and the emitter of a bipolar junction transistor. Therefore, the buffer 100 is effectively limited to operation in a buffer voltage range 210 defined by the difference in the first supply voltage rail 102 less a diode drop and the second supply voltage rail 104 plus a diode drop. Assuming that the diode drop is approximately 0.8 V, the buffer voltage range 210 extends from approximately 0.8 V to approximately +VBB-0.8 V as depicted in FIG. 2.
In low power applications, the loss of operating range, or lack of ability to swing rail-to-rail, due to the two diode drops creates a substantial limitation. For example, if the first supply voltage rail 102 is 3 V and the second supply voltage rail 104 is 0 V as shown, the maximum voltage range 208 becomes 3 V and the buffer voltage range 210 becomes approximately 1.4 V. In such a scenario, the buffer 100 would be incapable of providing the output signal and driving the load for more than half (approximately 1.6 V) of the 3 V maximum voltage range 208.
Therefore, what is needed is an output stage for an op amp having rail-to-rail swing capability so as to be suitable for use in a low voltage application and having a low output impedance so as to be suitable for mounting at the output of an IC.